Trenching equipment

ABSTRACT

A hand-held, self-contained motor-driven trenching device ( 100 ) for cutting a slit or trench within a substrate is based on a chain-saw motor unit together with a purpose-built attachment. An unmodified chain-saw drive unit provides both motive force and a holder, although purpose-built units may employ other forms of motive power. A chain-driven reduction drive matches the chain-saw drive sprocket ( 101 ) speed to a suitable toothed digging chain cutting speed; the reduction drive being coupled to a sprocket wheel ( 102 ) aligned at one end of a sturdy, hardened steel blade ( 106 ) over which a heavy roller chain ( 200 ) bearing soil-cutting teeth ( 203 ) is pulled. The resulting trench is about 30-35 mm wide, up to 1 meter deep, and of indefinite length. The invention provides an optimized solution to a need for burying narrow cables, fiber-optic conduits, water pipes, and the like well into the ground.

FIELD

This invention relates to portable motorised equipment for digging atrench into the ground, particularly hand-held equipment, and includesattachments for existing hand-held power units.

BACKGROUND

The power digging of holes or trenches is many times faster than manualdigging with hand tools. Because of the considerable horsepower neededto drive a chain or mechanical digger, its weight and size, and the needfor stable control of the digging bar or bucket, virtually allmechanical digging tools are mounted on a custom made vehicle. Thismakes them physically large, expensive, impractical, or uneconomic toown or to use for small jobs, and difficult to transport, typicallyrequiring a second vehicle to take them from place to place.

On many occasions about a house, a farm, or at construction sites, thereis a need to bury a relatively small conduit down to perhaps 1 meterdeep within the ground—typical conduit diameters being about 25 mm forexamples such as telephone cables, for water pipes about a farm, orsmall mains power cables. It is wasteful to dig a trench for such a thinconduit with hand tools which are usually about 150 mm wide or more, orto use one of the previously mentioned chain or mechanical diggers.

Observing the power and degree of control available from a chain sawstyle power unit, experiments have taken place to ascertain whether auseful and cost-effective trench digger could be created using achain-saw as a power unit. Chain-saws are widely distributed, therebeing about 25,000 chain-saws sold in New Zealand (about one per 120persons) per annum. It appears preferable to direct the inventiontowards providing an attachment capable of being reversibly attached toan existing chain-saw, for those who already possess a chain-saw. Theblade is usually held in place with two bolts.

A review of the prior art in this general area reveals: (1) U.S. Pat.No. 4,429,477 Tice et al, who describe a modified chain-saw, including agear reducer, for digging (and particularly deals with digger chaintooth design) (2) U.S. Pat. No. 4,651,449 Rose. (3) U.S. Pat. No.4,922,763 Ashworth where an ordinary chain-saw is provided with acatcher for dragged-up soil, to serve as a soil sampler, and (4) themore extreme U.S. Pat. No. 4,799,320 West et al (which provideshorizontally aimed augers to each side of the far end of a chain-sawblade). (5) U.S. Pat. No. 4,660,306 Bruce et al describes a trencherattachment for use on the arm of a hydraulic excavator. There is a groupof patents for digging chains for dedicated excavators. Example chainpatents are: U.S. Pat. No. 6,154,987 Rumer et al, U.S. Pat. No.4,924,609 Martin, U.S. Pat. No. 4,893,464 Thuerman, U.S. Pat. No.4,651,449 Rose (this chain is for a chain-saw), U.S. Pat. No. 3,913,979Strauss et al, and U.S. Pat. No. 3,846,922 Horton.

The problem to be solved might be stated as “to provide a cheap,portable powered trenching machine able to be held by a person while inuse”.

OBJECT

It is an object of this invention to provide means for the portable,motorised digging of a trench or at least to provide the public with auseful choice.

STATEMENT OF INVENTION

In a first broad aspect this invention provides a hand-held,self-contained motor-driven trenching device for cutting a slit within asubstrate, wherein the trenching device includes a driven digging chainin the form of an endless loop, having attached teeth, the chainsurrounding an outwardly and downwardly protruding cutting beam andbeing capable of being pulled along a side of the cutting beam by meansof a driven rotatable chain engagement means, and wherein the cuttingbeam is placed substantially in line with the centre of mass of thetrenching device, so that the weight of the trenching device fallsevenly on the cutting beam when in use to cut a slit in the substrate,and so that in use a person holding the motor-driven trenching devicedoes not experience undue sideways pressure.

Preferably the spacing along the shaft between the driven sprocket andthe primary chain receiving sprocket is sufficient to place the drivensprocket in line with the centre of gravity of the assembled chain-sawand attachment, so that in use a person holding the assembly does notexperience a sideways torque caused by an unbalanced mass.

Optionally that spacing along the shaft may be adjustable, so thatdifferent chain-saw drive units can be employed.

Preferably the driven sprocket is attached to the shaft using a key orspline.

In a second broad aspect this invention provides a for use in cutting agroove or trench in a substrate, wherein the trenching device iscomprised of a chain-saw motor drive unit and a trenching attachmentadapted for use with the chain-saw motor drive unit; said trenchingattachment being capable of repeated attachment to and removal from thechain-saw motor drive unit as a substitute for the original cuttingchain and saw blade of the chain-saw.

In a related aspect the trenching attachment includes coupling meanscapable of being firmly yet reversibly attached to the chain-saw driveunit by means of at least one fastener; said coupling means beingattached to a digging chain support means, comprising a first drivensprocket and a nose (idler) sprocket held apart by an elongated cuttingbeam lying in line with and between the sprockets, wherein the distancebetween the first driven sprocket and the nose sprocket is adjustable(so that a digging chain in the form of an elongated loop can betightened and held tightly between the sprockets and around theelongated beam), digging chain support means also comprising at leastone sliding surface along at least one side of said elongated beam,drive connection means to couple the first driven sprocket with a seconddriven sprocket capable of accepting the output of the chain-saw motorby means of an endless primary drive chain capable of being attached tothe powered sprocket of the chain-saw drive unit, said means comprisinga rotatable shaft supported in perpendicular relationship to theelongated beam, the shaft having the first driven sprocket affixed inline with the elongated beam, the shaft also serving to laterallydisplace the primary drive chain to one side of the digging chain, sothat the cutting beam may be placed substantially in line with thecentre of mass of the trenching device regardless of the position of thepowered sprocket of the chain-saw drive unit.

Preferably the trenching attachment further includes shaft speedreduction means, so that when in use the digging chain is pulled alongthe elongated beam at a velocity which is slower than that of theprimary drive chain while the motor of the chain-saw drive unit is ableto rotate at a preferred rate.

In a related aspect the second driven sprocket capable of accepting theoutput of the chain-saw motor has a larger circumference than that ofthe powered sprocket of the chain-saw drive unit.

Preferably a shaft speed reduction means is included within theaccessory, so that when in use the digging chain is pulled along theelongated beam at a velocity which is slower than that of the primaryendless chain, yet with more force, and a preferred reduction ofvelocity is to about 40% of initial velocity, although this factordepends on digging chain configuration and soil characteristics.

In a further related aspect the second driven sprocket has about threetines as many teeth as does the powered sprocket of the chain-saw driveunit.

In a third broad aspect the trenching attachment is further providedwith ground rest means, said ground rest means comprising one or morebars attached at one end to the trenching attachment, each bar includingintermediate ground contact areas, and each bar terminating in a handle.

Preferably each intermediate ground contact area comprises a wheel.

In a fourth broad aspect the attachment further includes a first guardcapable of enclosing the primary chain, when in use, and a second guardcapable of partially enclosing the first driven sprocket, so that whenusing the device a person is reasonably protected from injury and fromflying debris.

In a fifth broad aspect the invention provides an attachment furtherincluding means for sensing electric current flow, so that a person iswarned of electrical hazards derived from underground cables when usingthe device.

In a sixth broad aspect the invention provides a digging chain, thedigging chain being in the form of an endless loop of roller chain,provided with projecting teeth on one side; the teeth including aleft-hand, a right-hand and a central cutting type of tooth.

PREFERRED EMBODIMENT

The descriptions of the invention to be provided herein are given purelyby way of example and are not to be taken in any way as limiting thescope or extent of the invention.

DRAWINGS

FIG. 1: is a side view of the chain-saw attachment.

FIG. 2: shows detail of a digging chain.

FIGS. 3, 4, and 5: show sections through example tooth configurations ofa digging chain.

FIG. 6: shows another side of the chain-saw attachment.

FIG. 7: shows a cross-section of the chain-saw attachment.

FIG. 8: shows a skid for supporting and pulling the attachment when inuse.

FIG. 9: shows a dolly for supporting and pulling the attachment when inuse.

FIG. 10: shows a way to attach wheels to the attachment.

As foreshadowed in the Background, a convenient and portable digging andtrenching tool capable of cutting grooves in soil about 30-35 mm wide,up to 1 meter deep and of unrestricted length is provided by the presentinvention. Advantages of the present invention, which was developed asan attachment for use with the motor section of an ordinary chain-saw,will become apparent in the following description, with reference to theFigures.

The invention may be sold as a separate attachment for an existingchain-saw, preferably where the configuration of the existing chain-saw(in relation to positions of bolts 110, 111 for example) complies withestablished standards, so that assembly or disassembly can be carriedout from time to time by an end-user. Alternatively the invention can besold as a complete assembly together with a dedicated motor drive unitwhich for the sake of convenience (and completed field evaluation) maybe taken from a chain-saw manufacturer's stocks. Both the inventor'sprototype and the first batch of production models have had essentiallythe same constructional features as will be described herein.

In principle, the invention provides a novel attachment 100 capable ofbeng attached to an unmodified chain-saw drive unit 120 (which providesboth the motive force and a handle). The cutting chain and blade of theordinary chain-saw is unbolted and removed. The purpose-built attachmentincludes a chain-driven reduction drive (chain 601 and 22-tooth drivensprocket 102, in combination with the existing 7-tooth drive sprocket101 of the chain-saw) to better match the working chain-saw pulley speedto a suitable digging chain 200 cutting speed; the reduction drive beingcoupled along a shaft 103 to a driven sprocket wheel 104 mounted inalignment with one end of a sturdy, hardened steel blade or bar 106 overwhich the digging chain 200—an endless loop of heavy roller chainbearing soil-cutting teeth 201, 203, 501 is pulled. There is a nose(idling) sprocket and in this Example the sprocket—sprocket distance is820 mm. As is the case in an ordinary chain-saw the chain is pulledupward (toward the motor unit) underneath the steel bar. The inventionprovides an optimised solution to a need for burying narrow cables,water pipes and the like well into the ground.

The weight of the device is used to advantage. The weight tends to makethe device fall into the ground, so that continual engagement of thetool with the working material is assured. The inventors consider ituseful to make the weight of the device (chain-saw together with theattachment) hang “beneath” the point along the chain-saw grip bar whichis most conveniently held, in line with the middle of the chain-saw unititself, so that the person holding the grip bar does not have to exert asideways force. The invention includes (as shown in FIG. 7) anoffsetting of the line of the digging chain (along bar 106) from theline of the original chain-saw saw blade at one side of the drive unit,in line with the drive sprocket 101. This offset is provided for by theshaft 103 and it is clear that any convenient offset can be constructedeither at the time of manufacture or by way of a field modification. Inthe prototypes we use a 55 mm offset. The offset also serves to keep anydirt flung out by the digging chain 200 far from the drive pulley 101even though the drive pulley is preferably fully enclosed by a shieldprovided as part of the attachment. Guard 119 (FIGS. 6 and 7) helps tocontain the flung dirt. In detail, the entire attachment comprises thefollowing parts:

Attachment means comprising, at least for currently available chain-sawdrive units, the bar 112 including (for chain tightening purposes) anelongated aperture capable of accepting existing studs 110, 111protruding from the chain-saw drive unit. (FIGS. 1, 6) The drive shaftfrom the crankshaft and clutch holds the existing chain-saw sprocket101. Nuts on the studs 110 and 111 are used to fasten the attachment tothe chain-saw drive unit in the same manner as a chain-saw blade isnormally attached. The bar 112 is bolted (in this Example by the boltsthat hold the guard 119) to the remainder of the attachment andspecifically to the block 117. Hence, variations of the bar 112 can bedesigned and sold with the digging/trenching attachment to suit avariety of chain-saw drive units.

Drive means includes a first drive chain 601—like an ordinary chain-sawcutter chain but preferably with the cutting teeth removed—is wrappedaround existing chain-saw sprocket 101 and new sprocket 102—preferablyof a larger circumference in order to provide a reduction of speed. Thenew sprocket includes a circumferential array of sockets capable ofengaging with the teeth 603 projecting inwardly from chain 601, therebyproviding a positive drive. This first chain is preferably enclosedwithin a shield 118. Sprocket 102 is firmly attached to shaft 103 whichshaft is free to rotate within a pair of ball bearings 108 and 108 a(see FIG. 7). A significant sideways loading may be expected on thesebearings during use. In the prototypes, sprocket 102 has been made bylaser-cutting or otherwise shaping the perimeter of a steel disk thensandwiching it between two plain steel disks, welding the assemblytogether, and fitting the assembly onto the shaft using a left-handthread. At or near the other end of the shaft, a digger chain drivingsprocket 104 is attached by means of a coaxial cap screw 109 and thrustwasher, and in this Example, a key within a keyway milled into theshaft, in order to prevent inadvertent spinning. Both driven and nosesprockets are relieved sprockets, 9 mm thick and 73 mm outer diameter.

Digger chain support and moving means is provided by an elongated beam106, preferably made of a hardened steel (such as “Bizaloy”, about 50mm×10 mm), over which the roller chain 200 slides as shown in section inFIG. 3. The sliding area is of course at risk of abrasion from theworking material (the soil). At the remote end of the beam 106 there ismounted a nose sprocket wheel 105, held in line with and off the end ofthe beam by two bars 114 fastened in place by cap-screws 113, 113 aaccording to standard engineering practice. It is useful for the nosesprocket mounting to lie within the space “swept” by the cutting teethof the digging chain. A ball or roller bearing sealed against ingress ofdirt may be provided about the pivot 115 of the nose sprocket wheel,though such a bearing is not shown in these illustrations. Alternativelythese parts can be regarded as needing frequent replacement, or madeeasily greasable. The driven sprocket 104 is held firmly in line withand at an adjustable distance (for chain adjustment purposes) from thebeam 106 by means of bolts 107, 107 a preferably welded over the housingof the bearings 108, 108 a and hence to a mortice, keyway, or similarconstruction used to minimise twisting strains about the bolts 107, 107a by restricting possible freedom of the beam. Note that the beam is ofindefinite length as indicated by the curly lines 602.

Digger chain design was required because no small existing trencherchains were available for the 30-35 mm trench width created by thisinvention. We use a standard ⅝″ transmission chain. FIG. 2 shows at 200a roller chain having a number of teeth 201, 202, 203 of hardened steel(currently “Bizaloy”, 3 mm thick×20 mm long), welded at selected placesalong its length to outer connectors. As shown in FIGS. 3, 4, and 5, avariety of tooth shapes provides cutting edges at left, right, andcentre to tear or cut against the newly formed end of the trench. Aswill be apparent to a skilled reader, there is freedom to experimentwith tooth design, tooth frequency, attachment means, and possibly“scooping teeth” in relation to (a) soil types that may be encountered,and (b) characteristics (such as chain speed and power) of various motorunits. Excessive tooth length is undesirable.

Guards and enclosures: Guard 119 comprises a semicircular shieldparticularly intended to catch dirt flung from the chain-saw as it whipsaround the driven sprocket, or dirt generally pulled from the trench. Amodification of this guard could be used to collect and hence sample thesoil for purposes of farm fertiliser application monitoring. Enclosure118 (shown as a dotted line in FIG. 7) serves to keep the first drivingchain away from human hands, and protects that chain from dirt.

Unit support: For short runs, no support is required. If a worker is touse the device for an extended period without developing fatigue, skidsor wheels may be attached in order to support the weight of the unitplus its tendency to pull downwards, to act as a pivot point, to holdthe unit in case the user loses his footing or is otherwise interrupted,and to help guide the unit along a marked path. FIG. 8 shows a skid,made of pipes 801, 803 attached to the attachment at about the housingaround its shaft. The loop 802 may be placed around and behind theworker's back so that he can force the attachment along more easily,although we may prefer to use a rope optionally with a rubber pad behindthe user's back. One has to bear in mind the possibility that the unitbecomes electrically alive at an unpredictable moment. The outer curveat 803 acts as a fulcrum if the device is swung into the soil. In FIG.9, a similar support of pipes 902 is shown, having wheels 900, 901. InFIG. 10, a convenient way to attach wheels is shown in section; a curvedsteel shroud 1000 is shaped to accept the lower part of the semicircularguard 119 into aperture 1003 and be locked in place with hand screw1002. A tubular axle holder 1001 is welded across part of the shroud anda set of wheels joined by an axle is passed through the tube 1001. Arubber insulation layer may be used within this wheel mounting to helpminimise the size of the conducting body in case of cutting into a powercable.

Variations

Bar length: We prefer about 800 mm as a maximum bar length for use witha chain-saw drive unit of medium power/capacity. A larger drive unitcould be used with a bar of 1 meter or even 1.5 m length, where trenchesof that depth are considered useful. Note that the ends of the bar 106have been scalloped (116, FIG. 6) in order to extend the sliding surfacethat backs and supports the chain 200.

A purpose-built trench-digging tool may be constructed, comprisingpermanent assembly of the above attachment together with a source ofmotive power—such as an internal-combustion engine, a pneumatic motor, ahydraulic motor (perhaps driven off a farm tractor), or an electricmotor. Preferably the motor is built into a housing having convenienthandles and weight relief means.

Other speed reduction means: The driven sprocket 104 could be replacedby one having a reduced tooth count. There is room for a planetary gearsystem to be included along the axis of the shaft 103, although this maybe more expensive than simple use of a larger sprocket as 102, as shownhere. There may be cases where the extra reduction is required. A beltdrive could be used to collect power from the drive unit and avariable-speed belt transmission might be placed here. A low-speed driveunit may be made especially for this application. The amount ofreduction disclosed in this example is sufficient to allow the preferredchain-saw drive unit, with a chain as described herein, to run for anextended period with the internal combustion (petrol) engine at aneconomical, and well-cooled rate.

Trials: Continual development of the invention has produced numerousrefinements in the current Example as described herein. We haveexperimented with variations in cutter bar length, digger chain design,and power units. The current unit is regarded as being ergonomicallybalanced, has minimal vibration, and rapidly cuts a trench. In onetrial, using wheels, a 35 meter long, 30 mm wide and 450 mm deep trenchwas cut into a difficult wet clay soil in 20 minutes. Trial conditionson various substrates have included hard dry clay soils, moist soils,saturated wet clay soils, alluvial sandy soil, and across limestonegravel cattle races. A local electric power authority ran a trial andcut through tree roots without difficulty. The wheel support has beenfound very useful for longer trench runs.

Safety: is a known problem with any self-powered device having anexposed cutting area, especially for an ordinary chain-saw. Inparticular it is likely that an increasing number of already-buriedpower cables will be encountered by a user of this trench-digging deviceover time, unless stringent rules for marking and observing thepositions of previously laid cables are made and followed. A user shouldhold the device only by the insulated grips, should wear rubber or otherfully insulating boots, and should be aware that at any time thechain-saw and accessory may become electrically live as a result ofhaving sawn into an unsuspected live electric cable.

A leakage current sensor may be a useful addition. See FIG. 8. One formof sensor involves use of a remote ear peg 805, connected by wire 806 tothe chassis 120 of the invention 100 (which is a conducting unit),through a current-sensing module 807. In the event of any significantflow of current through the wire, the chain-saw ignition circuit isimmediately disabled and a visual and audible alarm 810 sounds. Userswill have been particularly instructed to treat the invention aselectrically live when this alarm condition occurs. The sensitivity ofthis device may be sufficient to detect relatively small electriccurrents flowing adjacent to an earthed conduit, as a result of earthloops or in particular the neutral currents relied on in three-phasecircuits or the like, or the sensitivity may be so low that only contactwith live conductors sets it off. Pulses arriving from electric fencesas used on New Zealand farms may require active rejection.

Commercial Benefits or Advantages

The invention is more convenient, more manoeuvrable, easily transportedwithout a special vehicle, and much cheaper than conventional motoriseddigging equipment

Many trenches need be only about 30-35 mm wide to accommodate a 25 mmcable or water-pipe and it is wasteful to dig a wider trench.

In situations where a wider, accurately cut trench is required theinvention may be used to create a pair of parallel cuts a desireddistance apart. A spade or the like is used to remove the soil betweenthe cuts.

This tool is useful for telephone installers and linesmen, electricians,builders, plumbers, landscapers, as well as farmers.

Because many outdoors workers already possess a chain-saw, it is usefulto provide an accessory that replaces the usual saw blade and cutterchain.

The lower speed of the digger chain is an advantage over prior-artdevices which used the unaltered chain-saw speed in a soil trencher.

The offset axis of the digger chain places the cutter bar at about thecentre of gravity of the unit, and separates thrown-out soil from theregion of the chain-saw drive chain.

It is advantageous to separate the high-wear digging chain from thedrive mechanism.

Although a number of preferred examples as described above have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions, and substitutions tothe apparatus and methods described herein are possible withoutdeparting from the scope and spirit of the invention as set forth in thefollowing claims.

We claim:
 1. A hand-held motorized trenching device for cutting a slitor trench into a substrate, comprising: a chain-saw motor unit; anelongated and outwardly extending cutting blade, the cutting bladeattached to the chain-saw motor unit; the cutting blade supporting adigging chain in the form of an endless loop surrounding the cuttingblade, the digging chain carrying a plurality of attached digging teeth;the digging chain being capable of being pulled along an edge of thecutting blade by a secondary drive wheel located at a proximal end ofthe cutting blade; the secondary drive wheel mounted on a second end ofa rotatably supported shaft; and a primary drive wheel mounted on afirst end of the shaft in line with a drive sprocket of the chain-sawmotor unit so that an endless drive chain can be placed over the primarydrive wheel and the drive sprocket, whereby in use, operation of themotor causes the digging chain to be pulled, a distance between theprimary drive wheel and the secondary drive wheel being such that thesecond drive wheel and the cutting blade are held in line with themiddle of the chain-saw unit, wherein a person holding a grip bar of thedevice does not have to exert a sideways force in order to counteract alack of balance of a weight of the trenching device, and the weight ofthe trenching device falls in line with an area on the grip bar of thechain-saw motor unit that is gripped by a user when in use.
 2. Amotorized trenching device as claimed in claim 1, wherein the trenchingdevice further includes a protective guard located between the diggingchain and the chain-saw motor unit, so that in use the guard trapsdebris flung off the chain.
 3. A motorized trenching device as claimedin claim 1, wherein relative diameters of the primary drive wheel andthe secondary drive wheel are selected, in relation to the drivesprocket of the chain-saw motor unit, so as to provide a reduction inspeed of the digging chain, as compared to a speed of a conventionalchain-saw chain directly attached to the drive sprocket of the chain-sawmotor unit, and so that the motor and the digging chain can operatetogether yet at respective optimum speeds.
 4. A motorized trenchingdevice as claimed in claim 3, wherein the relative diameters of theprimary drive wheel and the secondary drive wheel provide an about 3:1reduction in speed of the digging chain as compared to the speed of theconventional chain-saw chain directly attached to the drive sprocket ofthe chain-saw motor unit.
 5. A motorized trenching device as claimed inclaim 4, wherein the trenching device is further provided with groundrest means comprising one or more bars attached at one end to thetrenching device, each bar including intermediate ground contact areas.6. A trenching device as claimed in claim 5, wherein each bar terminatesin a handle.
 7. A trenching device as claimed in claim 5, wherein eachintermediate ground contact area comprises a wheel.
 8. A trenchingdevice as claimed in claim 1, wherein the trenching device furtherincludes a detector for sensing electric current flow so that a personis warned of electrical hazards derived from underground cables whenusing the device.
 9. An attachment for use in combination with astandard chain-saw motor unit, comprising, when combined, a hand-heldmotorized trenching device to be used by a person for cutting a slit ortrench into a substrate, the attachment comprising: an elongated andoutwardly extending cutting blade, the cutting blade adapted to bereversibly attached to the chain-saw motor unit; the cutting bladesupporting a digging chain in the form of an endless loop surroundingthe cutting blade, the digging chain carrying a plurality of attacheddigging teeth; the digging chain being capable of being pulled along anedge of the cutting blade by a secondary drive wheel located at aproximal end of the cutting blade; the secondary drive wheel mounted ona second end of a rotatably supported shaft; and a primary drive wheelmounted on a first end of the shaft in line with a drive sprocket of thechain-saw motor unit so that an endless drive chain can be placed overthe primary drive wheel and the drive sprocket, whereby in use,operation of the motor causes the digging chain to be pulled, a distancebetween the primary drive wheel and the secondary drive wheel being suchthat the second drive wheel and the cutting blade are held substantiallyin line with the middle of the chain-saw unit, wherein a person holdinga grip bar does not have to exert a sideways force in order tocounteract a lack of balance of a weight of the trenching device, andthe weight of the trenching device falls in line with an area on thegrip bar of the chain-saw motor unit that is gripped by a user when inuse.
 10. A motorized trenching device as claimed in claim 9, wherein thetrenching device further includes a protective guard located between thedigging chain and the chain-saw motor unit, so that in use the guardtraps debris flung off the chain.
 11. A motorized trenching device asclaimed in claim 9, wherein relative diameters of the primary drivewheel and the secondary drive wheel are selected, in relation to thedrive sprocket of the chain-saw motor unit, so as to provide a reductionin speed of the digging chain, as compared to a speed of a conventionalchain-saw chain directly attached to the drive sprocket of the chain-sawmotor unit, and so that the motor and the digging chain can operatetogether yet at respective optimum speeds.
 12. A motorized trenchingdevice as claimed in claim 11, wherein the relative diameters of theprimary drive wheel and the secondary drive wheel provide an about 3:1reduction in speed of the digging chain as compared to the speed of theconventional chain-saw chain directly attached to the drive sprocket ofthe chain-saw motor unit.
 13. A motorized trenching device as claimed inclaim 12, wherein the trenching device is further provided with groundrest means comprising one or more bars attached at one end to thetrenching device, each bar including intermediate ground contact areas.14. A trenching device as claimed in claim 13, wherein each barterminates in a handle.
 15. A trenching device as claimed in claim 14,wherein each intermediate ground contact area comprises a wheel.
 16. Atrenching device as claimed in claim 9, wherein the trenching devicefurther includes a detector for sensing electric current flow so that aperson is warned of electrical hazards derived from underground cableswhen using the device.